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Introduction:
A sophisticated cyberattack campaign is currently underway, targeting on-premises Microsoft Exchange servers that remain unpatched against known vulnerabilities. The attackers are leveraging a combination of publicly disclosed exploits to gain initial access, deploy web shells for persistent remote control, and ultimately harvest credentials while moving laterally across compromised networks. This campaign underscores the critical importance of timely patch management and proactive threat hunting.
Learning Objectives:
- Identify the specific vulnerabilities being exploited and their impact on Exchange servers.
- Learn to detect indicators of compromise (IoCs) such as web shells and unusual process activity.
- Implement step-by-step mitigation and hardening procedures for Exchange and associated infrastructure.
You Should Know:
1. Identifying the Attack Vector: ProxyShell and ProxyLogon
The current campaign heavily utilizes exploits from the “ProxyShell” and “ProxyLogon” families. These are not new zero-days but rather older, well-documented vulnerabilities that administrators have failed to patch.
- ProxyLogon (CVEs-2021-26855, 26857, 26858, 27065): This chain allows an unauthenticated attacker to execute arbitrary code on an Exchange server. The initial vector is typically CVE-2021-26855, a server-side request forgery (SSRF) vulnerability that bypasses authentication. Once this foothold is gained, attackers can write a file to the server.
- ProxyShell (CVEs-2021-34473, 34523, 31207): This is a similar, more recent chain that also leads to remote code execution through the autodiscover service.
Step‑by‑step guide to check your server:
First, verify if your server is patched against these specific vulnerabilities.
PowerShell (Run as Administrator on Exchange Server):
Check installed updates against known KBs for Exchange 2016/2019
Get-HotFix -Id KB5001779, KB5003435, KB5004780, KB5005338, KB5007012, KB5007013
A better method: Check the Exchange build number
Get-Command ExSetup.exe | ForEach-Object {$_.FileVersionInfo}
Compare the FileVersion (e.g., 15.2.858.5) against Microsoft's official build numbers table:
https://docs.microsoft.com/en-us/exchange/new-features/build-numbers-and-release-dates
If your build number is lower than the versions listed for the security updates from April 2021 onwards, you are vulnerable.
2. Detecting Web Shells Deployed Post-Exploitation
Once the attackers gain code execution, they deploy web shells to maintain persistent access. These are typically small scripts placed within the webroot of the Exchange Server, often masquerading as legitimate `.aspx` or `.ashx` files.
Linux Command-line Hunting:
If you have a compromised server, you can mount the Windows drive on a Linux analysis machine or use a live CD to scan for anomalies without relying on a potentially compromised OS.
Mount the Windows partition (assuming /dev/sda2 is the Windows drive) sudo mount /dev/sda2 /mnt/windows Search for recently created .aspx files in the Exchange web directories find /mnt/windows/inetpub/wwwroot -name ".aspx" -type f -mtime -30 -ls find /mnt/windows/Program\ Files/Microsoft/Exchange/ -name ".aspx" -type f -mtime -30 -ls 2>/dev/null Grep for common web shell patterns (base64 decoding, eval, etc.) grep -r -l "eval(base64_decode" /mnt/windows/Program\ Files/Microsoft/Exchange/ 2>/dev/null grep -r -l "System.Diagnostics.Process" /mnt/windows/Program\ Files/Microsoft/Exchange/ 2>/dev/null
Windows Command-line Hunting:
On a live, isolated system, use PowerShell to hunt for indicators.
Search for recently created or modified .aspx files in critical paths
Get-ChildItem -Path "C:\inetpub\wwwroot\aspnet_client","C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy","C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess" -Recurse -Filter .aspx | Where-Object {$_.LastWriteTime -gt (Get-Date).AddDays(-30)} | Select-Object FullName, LastWriteTime
Look for processes initiating network connections from the Exchange server
netstat -ano | findstr ESTABLISHED
Correlate the PID from netstat with running processes
tasklist | findstr [bash]
3. Analyzing Logs for Signs of Compromise
Web shells generate specific log entries. Focus on Exchange HTTP proxy logs and IIS logs.
Log Locations:
- IIS Logs: `C:\inetpub\logs\LogFiles\W3SVC1\`
– Exchange HTTP Proxy Logs: `C:\Program Files\Microsoft\Exchange Server\V15\Logging\HttpProxy\`
PowerShell Log Analysis:
Check IIS logs for requests to可疑的 .aspx pages that aren't normally accessed Select-String -Path "C:\inetpub\logs\LogFiles\W3SVC1\u_ex.log" -Pattern "GET ..aspx" | Select-Object -First 20 Look for anomalies like large POST requests to PowerShell virtual directories Get-Content "C:\Program Files\Microsoft\Exchange Server\V15\Logging\HttpProxy\PowerShell.log" | Select-String "POST" | Select-Object -First 20
Key Indicator: Look for `200` status codes for `.aspx` files in the ecp, owa, or `powershell` virtual directories that you do not recognize as legitimate.
4. Mitigation: Patching and Network Segmentation
The primary mitigation is patching, but while patching, you must contain the spread.
Step 1: Isolate the Exchange Server.
On the server’s firewall or the network switch, block all inbound and outbound traffic except for essential management and mail flow. This prevents the attacker from exfiltrating data or using the server as a pivot point.
Windows Firewall: Block all outbound except to your management subnet and mail gateways (Example: Block all, then allow specific IPs - execute with extreme caution) netsh advfirewall set allprofiles firewallpolicy blockinbound,blockoutbound netsh advfirewall firewall add rule name="Allow DNS" dir=out protocol=udp remoteport=53 action=allow netsh advfirewall firewall add rule name="Allow SMTP to Mail Gateway" dir=out protocol=tcp remoteport=25 remoteip=192.168.1.100 action=allow
Step 2: Apply the Latest Cumulative Update (CU).
Patching Exchange is not just about installing a security update; it often requires installing the latest Cumulative Update. Download the latest CU for your version from the Microsoft Download Center and apply it. This will automatically install all previous security fixes and overwrite system files, potentially removing web shells placed in system directories.
5. Hardening Exchange and Active Directory
Preventing future attacks requires hardening the environment.
Disable Legacy Protocols and Authentication:
Many exploits target older, less secure protocols.
Disable Basic Authentication across Exchange virtual directories Get-OwaVirtualDirectory | Set-OwaVirtualDirectory -BasicAuthentication $false Get-EcpVirtualDirectory | Set-EcpVirtualDirectory -BasicAuthentication $false Get-ActiveSyncVirtualDirectory | Set-ActiveSyncVirtualDirectory -BasicAuthentication $false Enable MFA for all administrative accounts in Azure AD/On-Prem.
Principle of Least Privilege:
Ensure that service accounts and user accounts have the minimum required privileges. Attackers often use compromised Exchange servers to dump Active Directory credentials via techniques like DCSync.
Linux Command to check for DCSync attempts (on a SIEM or Domain Controller):
Look for Event ID 4662 (An operation was performed on an object) in Windows Event Logs forwarded to a Linux SIEM. grep "4662" /var/log/remote/.log | grep "DS-Replication-Get-Changes" | grep -v "All"
This indicates a request for replication, which is a sign of DCSync.
What Undercode Say:
- Patching is non-negotiable: The exploitation of years-old vulnerabilities highlights a massive gap in vulnerability management. An unpatched internet-facing Exchange server is a ticking time bomb.
- Defense in depth is critical: Web shells are a symptom of a deeper failure. Network segmentation, application control, and proper logging could have detected and blocked this attack at multiple stages, even if the initial vulnerability was present.
Prediction:
We will see a continued surge in automated scanning for these Exchange vulnerabilities as ransomware groups and initial access brokers commoditize this attack vector. Organizations that fail to implement automated patch management and continuous monitoring for web shells will face an increasing likelihood of a full-scale ransomware deployment, moving from data theft to operational disruption.
What Undercode Say:
- The sophistication of the initial exploit chain is less important than the simplicity of the fix. Apply updates immediately.
- Incident response must now assume that a compromised Exchange server means compromised credentials for the entire domain.
- The shift to cloud-based email solutions like Exchange Online will accelerate, as organizations seek to offload the burden of this critical patching responsibility to Microsoft.
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